Considering your app is at https://example.herokuapp.com/
, and assuming you own the domain www.example.com
as stated, follow the following steps to have your app running on your custom domain:
Using the Heroku CLI
- Add a custom domain with a subdomain
$ heroku domains:add www.example.com
- Add a custom root domain
$ heroku domains:add example.com
On namecheap
After signing into your account
- Select Domain List from the left side menu and click the Manage button next to your domain:

- Navigate to the Advanced DNS tab and click the Add New Record button in the Host Records section:

- To use
www.example.com
and example.com
, the following host records should be created:
URL Redirect Record
for @
to forward to http://www.example.com
(http://www.yourdomain.tld
in the picture)
CNAME Record
for www
mapped to your Heroku Domain www.example.com.herokudns.com.
(www.yourdomain.tld.herokudns.com.
in the picture)

- (Optional) To use any other subdomain, like
app.example.com
and example.com
, you should create the following records:
URL Redirect Record
for @
to forward to http://app.example.com
(http://app.yourdomain.tld
in the picture)
CNAME Record
for app mapped to your Heroku Domain app.example.com.herokudns.com.
(app.yourdomain.tld.herokudns.com.
in the picture)

That's it! Now your app should be running at www.example.com
.
For more information on the namecheap side you can check this link.